1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording method in which a magnetic recording medium is irradiated with light, thereby anisotropic magnetic field of the medium is lowered, then, magnetic recording is performed. Further, the present invention relates to a thermally-assisted magnetic recording head which writes data by using the thermally-assisted magnetic recording method.
2. Description of the Related Art
As the recording density of a magnetic recording apparatus becomes higher, as represented by a magnetic disk apparatus, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic microparticles gathered together, and each of the magnetic microparticles has an approximately single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic microparticles. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic microparticles and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic microparticles. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, the intensity of write field generated from the thin-film magnetic head is limited by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. Therefore, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium, where data is to be written; just after that, writing is performed by applying write field to the heated portion.
As generally-known thermally-assisted magnetic recording techniques, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894 disclose a method in which a magnetic recording medium is heated by using near-field light generated from a near-field light generator that is a conductive plate, so-called a plasmon antenna.
However, when such a plasmon antenna is used to implement thermally-assisted magnetic recording, a difficult problem could arise as described below.
While a plasmon antenna converts the received light to near-field light as described above, it is known that the light use efficiency is not so high; most part of the applied light changes to thermal energy in the plasmon antenna. Here, the size of the plasmon antenna is set to a value less than or equal to the wavelength of the light, thus its volume is very small. Accordingly, the thermal energy heats the plasmon antenna to an extremely high temperature; in some cases, the temperature of the plasmon antenna reaches approximately 500° C. (degrees Celsius). Such temperature rise causes the plasmon antenna to thermally expand and protrude from the opposed-to-medium surface toward a magnetic recording medium. As a result, the end, which reaches the opposed-to-medium surface, of a read head element for reading data signal or servo signal from the magnetic recording medium can become relatively far apart from the magnetic recording medium. If this is the case, it will be difficult to properly read the servo signal during writing in which the plasmon antenna is used to irradiate the magnetic recording medium with near-field light. In addition, the electrical resistance of the plasmon antenna increases to a considerably high value at such extremely high temperature. This means that the light use efficiency of the plasmon antenna described above can be further degraded because of increased thermal disturbance of free electrons in the plasmon antenna.
Another problem could arise in the case of combining the plasmon antenna and the main magnetic pole of a write head element. In a thermally-assisted magnetic recording, a plasmon antenna must be disposed sufficiently close to a main magnetic pole, whether on the trailing side or on the leading side of the main magnetic pole. Actually, the thermally-assisted magnetic recording that uses the plasmon antenna applies thermal-dominant technique in which spatial resolution of record bits depends on the spot diameter of near-field light. Therefore, temperature gradient adjacent to the irradiating center of near-field light becomes significantly large. While, magnetic-field gradient of write field generated from the main magnetic pole is set to be considerably large according to the higher recording density. As a result, in writing record bits, the irradiating center of near-field light, or the plasmon antenna, is required to be sufficiently close to the main magnetic pole in order to obtain write field with sufficient intensity near the irradiating center.
However, the plasmon antenna generates near-field light by receiving light that has propagated through a waveguide structure. The waveguide structure generally consists of a core region having a high refractive index and a clad region having a lower refractive index which surrounds the core region. To keep function as the waveguide, the thickness of each region is set comparable with the wavelength of the light. As a result, it is difficult that the plasmon antenna, which is provided so as to be opposed to the core region at the end on the opposed-to-medium surface side of the waveguide structure, is disposed sufficiently close to the main magnetic pole with a distance less than the wavelength of the light.
From above-described considerations, a thermally-assisted magnetic recording without using a near-field light generator such as a plasmon antenna is expected. For example, U.S. Pat. No. 6,636,460 B2 discloses a magnetic recording apparatus in which a semiconductor light-emitting element is provided on the leading side of the main magnetic pole instead of a plasmon antenna. The apparatus heats a portion of the magnetic recording medium by laser light spot generated from the semiconductor light-emitting element to lower coercive force of the portion. Then, recording is performed by applying magnetic field generated from the main magnetic pole to the portion with lowered coercive force. In the recording, a reversing point of magnetization in which the intensity of coercive force is equal to the intensity of write field is set in the magnetic pole region; thus, the recording is performed by positively utilizing the temperature gradient instead of the magnetic field gradient.
However, it is difficult to realize a satisfying thermally-assisted magnetic recording by just using a laser light spot with a large diameter. For example, in the case that the laser light spot is located on the leading side of the main magnetic pole, the magnetization transition of record bits is eventually decided under the condition that both of the temperature and magnetic field gradients are small. Therefore, it becomes very difficult to achieve a high linear recording density. While, in the case that the laser light spot is located on the trailing side of the main magnetic pole, record bits may be damaged, because the magnetization transition regions of record bits, which have once been decided, are disturbed by being exposed to higher temperature after the decision.
Further, even in the case of utilizing the laser light spot with a large diameter, a sufficient intensity of write field must be applied to the center of the laser light spot and its neighborhood in order to write record bits. For this purpose, the core region of the waveguide structure is needed to be sufficiently close to the main magnetic pole. Here, the present inventors are currently developing a writing method in which the distance between a main magnetic pole and a trailing shield that is placed on the trailing side from the main magnetic pole is set to be sufficiently small, and writing is performed by using a steep magnetic-field gradient that exists near the edge positioned on the opposed-to-medium surface and on the trailing side of the main magnetic pole. Therefore, the center of the laser light spot of the waveguide structure located on the leading side from the main magnetic pole is required to be much closer to the edge on the trailing side of the main magnetic pole; thus the distance between the waveguide structure and the main magnetic pole must be much smaller.
However, the smaller distance between the core region and the main magnetic pole is equivalent to a significantly thinner clad region or to the removal of the clad region. This may cause a problem that the light propagating through the core region can be easily absorbed into the main magnetic pole. The light propagating through the core region should reach the magnetic recording medium with a loss as low as possible to realize a satisfying thermally-assisted magnetic recording. Therefore, this absorption must be avoided as much as possible.